Method and a device for removing mercury from a process gas

ABSTRACT

A method of removing mercury from a process gas by means of a sorbent and a filter ( 10 ) involves applying said sorbent to at least one filtering surface ( 12 ) of the filter ( 10 ). A first parameter, which is indicative of the amount of mercury that needs to be removed in said filter ( 10 ), and a second parameter, which is indicative of the amount of material that has been collected on said filtering surface ( 12 ), are measured. A measured value of said first parameter is compared to a mercury set point. When said measured value of said first parameter is higher than said mercury set point, the cleaning of said filtering surface ( 12 ) is delayed, compared to the point in time suggested by a measured value of said second parameter.

This is a divisional application claiming priority to pendingapplication Ser. No. 13/124,904 having a filing date of Oct. 16, 2009,which claims priority to International Application No. PCT/EP2009/063539having an International Filing Date of Oct. 16, 2009, which claimspriority to EP Application No. 08167051.5 having a filing date of Oct.20, 2008, incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a method of removing, at least partly,mercury from a process gas by means of a mercury absorbing sorbent and afilter having at least one filtering surface.

The present invention further relates to a gas cleaning system, which isoperative for removing, at least partly, mercury from a process gas, thegas cleaning system comprising a sorbent supply system which isoperative for supplying a mercury absorbing sorbent to said process gas,and a filter having at least one filtering surface and being operativefor collecting said sorbent.

BACKGROUND OF THE INVENTION

In the combustion of a fuel, such as coal, oil, peat, waste, etc., in acombustion plant, such as a power plant, a hot process gas is generated,such hot process gas containing, among other components, mercury, Hg.Since mercury is hazardous to the human health and to the environment itis usually necessary to remove mercury from the process gas before itcan be discharged into the ambient air. The removal of mercury is oftenaccomplished by mixing the hot process gas with a solid sorbent, such aspulverized activated carbon, which adsorbs the mercury and which canthen be removed from the hot process gas in a filter, such as a baghouse.

U.S. Pat. No. 5,505,766 describes a gas cleaning system in which asorbent is supplied to a bag house from a silo. The sorbent forms asorbent layer on filter bags of the bag house. When a sufficiently thicksorbent layer has been formed on the filter bags, the hot process gas isadmitted to the bag house. When the removal of mercury decreases acompartment of the bag house is taken off-line, such that the mercuryloaded sorbent can be removed, and new sorbent can be added to the bags,before the process gas is again allowed to enter the compartment inquestion.

While the gas cleaning system of U.S. Pat. No. 5,505,766 ensures that alayer of sorbent will always be available on the filter bags for theremoval of mercury from the process gas, it is also a complicatedprocess that requires an advanced control and expensive equipment.Furthermore, the bag house must be over-sized compared to normaloperation, to allow that one compartment at a time is taken off-line forcleaning.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of cleaning aprocess gas, which method is more effective for removing mercury fromsuch process gas compared to the prior art method.

This object is achieved by means of a method of removing, at leastpartly, mercury from a process gas by means of a mercury absorbingsorbent and a filter having at least one filtering surface, the methodbeing characterised in

applying said sorbent to said at least one filtering surface,

measuring at least one first parameter, which is indicative of theamount of mercury that needs to be removed in said filter,

measuring at least one second parameter, which is indicative of theamount of material that has been collected on said filtering surface andwhich is utilized for determining when the filtering surface should becleaned,

comparing a measured value of said first parameter to a mercury setpoint, and

delaying, when said measured value of said first parameter is indicatingan amount of mercury which is higher than the amount of mercuryindicated by said mercury set point, the cleaning of said filteringsurface, compared to the point in time suggested by a measured value ofsaid second parameter.

An advantage of this method is that the mercury removal capacity of thefilter can be increased, temporarily, without increasing the sorbentconsumption. The measurement of the first parameter makes it possible toeffect such increased mercury removal capacity when needed.

According to one embodiment said mercury set point is related to theconcentration of mercury in the process gas upstream of the filter, saidfirst parameter being related to the concentration of mercury in theprocess gas upstream of the filter. The amount of mercury that needs tobe removed in the filter is related to the amount of mercury in theprocess gas that enters the filter. Measuring the concentration ofmercury in the process gas upstream of the filter thus provides afeed-forward input for determining the required mercury removal capacityof the filter.

According to one embodiment said mercury set point is related to theconcentration of mercury in the process gas downstream of the filter,said first parameter being related to the concentration of mercury inthe process gas downstream of the filter. The concentration of mercurydownstream of the filter is an indication of whether the filter operatesin a sufficiently efficient manner, with regard to emission limits.Measuring the concentration of mercury in the process gas downstream ofthe filter thus provides a feed-back input for determining the requiredmercury removal capacity of the filter.

According to one embodiment said first parameter is related to theconcentration of mercury both upstream and downstream of the filter.Such a first parameter makes it possible to control the cleaning of thefiltering surfaces more accurately, by taking into account both thefeed-forward information about how much mercury that needs to beremoved, and the feed-back information about how well the filter managesto remove the mercury.

In accordance with one embodiment the concentration of mercury ismeasured both upstream and downstream of the filter, at least one ofsuch measured concentrations being utilized for determining said firstparameter, at least the other of such measured concentrations beingutilized for determining a third parameter, which is utilized forcontrolling the supply of said sorbent. An advantage of this embodimentis that the control system can control both the cleaning of thefiltering surfaces of the filter, and the dosage of sorbent to obtainthe most efficient removal of mercury, with regard to consumption ofenergy and sorbent.

According to one embodiment said second parameter is a pressure dropover the filter, the cleaning of the filtering surface being initiatedwhen said second parameter exceeds a first pressure drop set point insituations when the measured value of said first parameter indicates anamount of mercury which is equal to, or lower than, the amount ofmercury indicated by said mercury set point, and being initiated whensaid second parameter exceeds a second pressure drop set point, whichrefers to a higher pressure drop than the first pressure drop set point,when said measured value of said first parameter indicates an amount ofmercury which is higher than the amount of mercury indicated by saidmercury set point. An advantage of this embodiment is that cleaning ofthe filter is allowed to start at a lower pressure drop when theconcentration of mercury in the process gas is low, such that the energyconsumed, including, e.g., fan power, by the filter is low. When theconcentration of mercury is high in the process gas, the cleaning isdelayed, such that increased mercury removal is effected. The pressuredrop functions as an indirect indicator of the removal efficiency of thefilter, since there is a relation between the pressure drop and thethickness of the dust cake, and a similar relation between the thicknessof the dust cake and the mercury removal efficiency. It will beappreciated that a measure, such as the filter flow resistance,providing equivalent information on the amount of material existing onthe filtering surface, may be utilized as alternative to the pressuredrop.

According to one embodiment said cleaning of said filtering surface isperformed at a first cleaning intensity when said measured value of saidfirst parameter is indicating an amount of mercury which is higher thanthe amount of mercury indicated by said mercury set point, and at asecond cleaning intensity, being higher in cleaning efficiency than saidfirst cleaning intensity, when said measured value of said firstparameter is indicating an amount of mercury which is equal to, or lowerthan, the amount of mercury indicated by said mercury set point. Anadvantage of this embodiment is that in situations of high requirementfor mercury removal the cleaning of the filtering surfaces is made in aless efficient way. In this manner a part of the dust cake remains onthe filtering surfaces also after the cleaning, such that mercuryremoval immediately after such cleaning remains on a high level.

An further object of the present invention is to provide a gas cleaningsystem which is operative for removing mercury from a process gas andwhich is more efficient compared to the prior art gas cleaning systems.

This object is achieved by means of a gas cleaning system which isoperative for removing, at least partly, mercury from a process gas, thegas cleaning system comprising a sorbent supply system which isoperative for supplying a mercury absorbing sorbent to said process gas,and a filter having at least one filtering surface and being operativefor collecting said sorbent, the gas cleaning system being characterisedin further comprising a control unit which is operative for controllingwhen said filtering surface is to be cleaned from material that has beencollected thereon, said control unit further being operative forreceiving a first signal referring to a measurement of at least onefirst parameter, which is indicative of the amount of mercury that needsto be removed in said filter, and a second signal referring to ameasurement of at least one second parameter, which is indicative of theamount of material that has been collected on said filtering surface andwhich is utilized for determining when the filtering surface should becleaned, the control unit further being operative for comparing ameasured value of said first parameter to a mercury set point, and fordelaying, when said measured value of said first parameter is indicatingan amount of mercury which is higher than the amount of mercuryindicated by said mercury set point, the cleaning of said filteringsurface, compared to the point in time suggested by a measured value ofsaid second parameter.

An advantage of this gas cleaning system is that it is effective forremoving mercury, also in a situation of varying mercury concentrations,a situation in which the prior art gas cleaning systems would require asubstantial increase in the sorbent supply. Thus, the present gascleaning system provides for a low sorbent supply, and, consequently,also a low formation of waste products, without increasing the emissionof mercury.

According to a further aspect of the present invention said method ofremoving, at least partly, mercury from a process gas by means of amercury absorbing sorbent and a filter having at least one filteringsurface is characterised in

applying said sorbent to said at least one filtering surface,

measuring at least one first parameter, which is indicative of theamount of mercury that needs to be removed in said filter,

measuring at least one second parameter, which is indicative of theamount of material that has been collected on said filtering surface andwhich is utilized for determining when the filtering surface should becleaned,

comparing a measured value of said first parameter to a mercury setpoint, and

cleaning said filtering surface at a first cleaning intensity when saidmeasured value of said first parameter is indicating an amount ofmercury which is higher than the amount of mercury indicated by saidmercury set point, and at a second cleaning intensity, being higher incleaning efficiency than said first cleaning intensity, when saidmeasured value of said first parameter is indicating an amount ofmercury which is equal to, or lower than, the amount of mercuryindicated by said mercury set point.

An advantage of this method is that the mercury removal capacity of thefilter can be increased, temporarily, by leaving more of the sorbentremaining on the filtering surface after cleaning due to a less intensecleaning of the filtering surface, without increasing the sorbentconsumption. The measurement of the first parameter makes it possible toeffect such increased mercury removal capacity when needed.

Further objects and features of the present invention will be apparentfrom the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theappended drawings in which:

FIG. 1 is a schematic side view of a power plant.

FIG. 2 is a flow diagram and illustrates a method of controlling thecleaning of filter bags of a bag house.

FIG. 3 is a flow diagram and illustrates an alternative method ofcontrolling the cleaning of filter bags of a bag house.

FIG. 4 is a schematic diagram, and illustrates the effect of operating agas cleaning system in accordance with the method of FIG. 2

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic side view and illustrates a power plant 1, as seenfrom the side thereof. The power plant 1 comprises a boiler 2. Duringcombustion of a fuel, such as coal or oil, a hot process gas, oftenreferred to as a flue gas, is generated in the boiler 2. The flue gas,which contains polluting substances, including dust particles andmercury, leaves the boiler 2 via a gas duct 4. The gas duct 4 isoperative for forwarding the flue gas to a primary dust collector 6,which is optional, in the form of, e.g., an electrostatic precipitator,an example of which is described in U.S. Pat. No. 4,502,872, or a baghouse, an example of which is described in U.S. Pat. No. 4,336,035. Theprimary dust collector 6 is operative for removing the major amount ofdust particles from the flue gas.

A gas duct 8 is operative for forwarding the flue gas from the primarydust collector 6 to a secondary dust collector 10. The secondary dustcollector 10 is a fabric filter, by which is meant that the flue gas isforced to pass through a filtering surface formed from a fabricmaterial. The secondary dust collector 10 illustrated in FIG. 1 is afabric filter of the so-called bag house type, which comprises aplurality of filter bags 12 made of a textile fabric. The fabric of thefilter bags 12 form filtering surfaces by means of which particulatematerial can be removed from the flue gas. A detailed description of anexample of a bag house can be found in U.S. Pat. No. 4,336,035. The baghouse 10 has three compartments 14, 16, 18. Each such compartment 14,16, 18 may comprise typically 2 to 20 000 filter bags 12. As can be seenfrom a reference to FIG. 1, the duct 8 is operative for supplying aseparate flow of flue gas to each of the compartments 14, 16, 18.

The flue gas passes through the textile fabric material of therespective filter bag 12, such that dust particles contained in the fluegas is captured on the outside of the bags 12. When passing through thefabric material of the filter bags 12 also very small dust particles areremoved from the flue gas, resulting in a very efficient removal of dustparticles. The cleaned flue gas enters a clean gas plenum 20, 22, 24 ofthe respective compartment 14, 16, 18. A clean gas duct 26 is operativefor forwarding the cleaned flue gas from each of the respective plenums20, 22, 24 to a stack 28, which is operative for discharging the cleanedflue gas to the ambient air.

A sorbent storage silo 30 is operative for containing a sorbent which issuitable for adsorbing mercury, in particular mercury in gaseous form. Asuitable sorbent could be activated carbon or coke in powdered form. Asorbent supply duct 32 is operative for forwarding the sorbent from thesilo 30 to the duct 8, in which the sorbent is mixed with the flue gas.

The sorbent, being a particulate material, will be collected on thefabric of the filter bags 12. The fabric material supports the formationof a dust cake on the outer face of each of the filter bags 12. Such adust cake, which will contain sorbent supplied from the silo 30, is veryefficient in removing mercury from the flue gas, because the flue gashas to pass through the dust cake. When passing through such a dustcake, containing the sorbent, the chance is high that mercury containedin the flue gas is adsorbed in the dust cake.

The dust cake formed on the respective filter bag 12 will increase theflue gas pressure drop over the bag house 10. Thus, it will be necessaryto perform intermittent cleaning of the filter bags 12. Each compartment14, 16, 18 is provided with a tank 34, 36, 38 containing pressurizedgas, usually in the form of pressurized air. The pressurized air may besupplied, as air pulses, via pipes 40 to the filter bags 12. Such airpulses forces the filter bags 12 to expand and causes the dust cakes toleave the bags 12 and fall into a respective hopper 42, 44, 46 fromwhich the collected dust particles may be transported away for furthertreatment, disposal, etc.

A control unit 48 is operative for controlling when the filter bags 12should be cleaned. Each compartment 14, 16, 18 is provided with apressure transducer which measures the pressure drop over the filterbags 12 of that specific compartment 14, 16, 18. A signal is sent fromeach of the pressure transducers to the control unit 48. For reasons ofclarity of illustration only the pressure transducer 50 of the firstcompartment 14 is illustrated in FIG. 1.

A first mercury analyser 52 is operative for measuring the concentrationof mercury in the flue gas in the duct 8, upstream of the bag house 10.The first mercury analyser 52 thus measures the concentration of gaseousmercury, Hg, in the flue gas before the sorbent has had any effect. Asecond mercury analyser 54 is operative for measuring the concentrationof gaseous mercury in the flue gas in the clean gas duct 26, downstreamof the bag house 10. The second mercury analyser 54 thus measures theconcentration of mercury in the cleaned flue gas. The mercury analysers52, 54 are operative for sending signals to the control unit 48.Preferably measurements are performed by the mercury analysers 52, 54,and corresponding signals are sent to the control unit 48, at least onceevery 30 minutes to obtain a quick response to changing conditions. Thecontrol unit 48 takes the signals from the pressure transducers, ofwhich only one transducer 50 is illustrated in FIG. 1, and the signalsfrom the mercury analysers 52, 54 into account when controlling theoperation of the tanks 34, 36, 38, i.e., when controlling the cleaningof the filter bags 12 of the respective compartment 14, 16, 18 by meansof pulsing, in a manner which will be described in more detailhereinafter. Furthermore, the control unit 48 may also control thesupply of sorbent from the sorbent silo 30 in a manner which will bedescribed in more detail hereinafter.

As described hereinbefore the control unit 48 receives a signal from thepressure transducer 50 which indicates the present pressure drop overthe filter bags 12 of the first compartment 14. This measured pressuredrop can be denoted DP. The control unit 48 further works with two setpoints for pressure drop. A first pressure drop set point, which can bedenoted DPHigh, indicates a pressure drop above which a cleaning of thefilter bags 12 is desirable, since a pressure drop above DPHigh isassociated with an increased energy consumption for forcing the flue gasthrough the filter bags 12 of the compartment 14. A second pressure dropset point, which can be denoted DPHighHigh, indicates a pressure drop atwhich a cleaning of the filter bags 12 must be initiated, also forreasons of such high pressure drop affecting the mechanical integrity ofthe bag house 10. The table 1 below indicates the basis for cleaning thebags 12 at different measured pressure drops, DP:

TABLE 1 Cleaning of filter bags, in relation to pressure drop DPrelation to set points DPHigh < DP < DP < DPHigh DPHighHigh DP >DPHighHigh Cleaning of No Depends also on Yes filter bags otherconditions

As described hereinbefore the control unit 48 also receives a signalfrom the first mercury analyser 52. The measured concentration ofmercury in the flue gas in the duct 8 can be denoted HGIN. The controlunit 48 further works with a mercury set point for the mercuryconcentration upstream of the bag house 10. Such a mercury set pointcould be equal to the average concentration of mercury in the flue gas,upstream of the bag house 10, i.e., in the duct 8, and could be denotedHGMEAN. The control unit 48 utilizes the measured concentration HGIN asa first parameter and compares such concentration to the mercury setpoint HGMEAN. When HGIN is less than or equal to HGMEAN the cleaning ofthe filter bags 12 is based only on the pressure drop DP. Hence, if HGINis equal to, or less than, HGMEAN, the filter bags 12 are cleaned, bymeans of pulsing them by pressurized air as described hereinbefore, assoon as the measured pressure drop DP, such pressure drop being utilizedas a second parameter, exceeds DPHigh. Such an approach results in thelowest possible energy consumption, and a sufficient removal of mercury.

However, in situations where the measured concentration of mercury HGINis higher than the mercury set point HGMEAN it is desirable to maintaina comparably thick dust cake on the filter bags 12, since such a thickdust cake is more efficient for removing mercury from the flue gascompared to a thin dust cake. Hence, in such situations, where the inletconcentration of mercury, i.e., HGIN, is comparably high, the controlunit 48 delays the cleaning of the filter bags 12, such that thecleaning of the filter bags 12 does not start until the measuredpressure drop DP exceeds the second pressure drop set point DPHighHigh.Thus, in situations of high inlet concentration of mercury a higherpressure drop over the bag house 10 is accepted, since such higherpressure drop also results in the filter bags 12 having on them athicker dust cake, which contains sorbent and which is effective forremoving such high concentration of mercury, without requiring anincreased dosage of sorbent from the silo 30. The pressure drop DPmeasured over the bag house 10 thus functions as an indirect indicatorof the removal efficiency of the bag house 10, since there is a relationbetween the pressure drop DP and the thickness of the dust cake on thefilter bags 12, and a similar relation between the thickness of the dustcake on the filter bags 12 and the mercury removal efficiency of the baghouse 10.

The table 2 below indicates the different decisions on the cleaning thatare taken by the control unit 48:

TABLE 2 Decisions taken by the control unit 48 HGIN relation to setpoint HGIN < or = HGMEAN HGIN > HGMEAN Cleaning of As soon as DP >DPHigh Not until DP > DPHighHigh filter bags

In this way the gas cleaning system, comprising the bag house 10 and thesorbent silo 30, is operative for effectively removing mercury from theflue gas also in situations where the inlet concentration of mercury ishigher than normal.

FIG. 2 illustrates in a graphical manner the method, describedhereinbefore, according to which the control unit 48 controls when thefilter bags 12 of the first compartment 14 of the bag house 10 should becleaned by means of pulsing them. In a first step, denoted 60 in FIG. 2,the control unit 48 has received a signal from the first mercuryanalyser 52 about the measured concentration of mercury in the flue gasin the duct 8. The control unit 48 checks if the measured concentrationof mercury, HGIN, is higher than the average inlet concentration ofmercury, HGMEAN. If the answer in the first step 60 is “YES”, then thecontrol unit 48 proceeds to a first alternative second step, denoted 62in FIG. 2, in which the control unit 48 checks whether the measuredpressure drop DP is higher than the second pressure drop set pointDPHighHigh. If the answer in the first step 60 is “NO”, then the controlunit 48 proceeds to a second alternative second step, denoted 64 in FIG.2, in which the control unit 48 checks whether the measured pressuredrop DP is higher than the first pressure drop set point DPHigh.

If the outcome of the check in the first alternative second step 62 is“NO”, then the control unit 48 returns to the first step 60 withoutperforming any cleaning of the filter bags 12. In accordance with thefirst step 60 a new measured mercury concentration HGIN is compared tothe average mercury concentration HGMEAN. Thus, in such a situation, thedust cake remains on the filter bags 12 for an extended period of time,to increase the mercury removal capacity of the bag house 10, withouthaving to add more sorbent than under normal mercury concentrations. Ifthe outcome of the check in the first alternative second step 62 is“YES”, then the control unit 48 proceeds to a first alternative thirdstep, denoted 66 in FIG. 2, in which the bags 12 are cleaned.Preferably, the first alternative third step 66 includes a partialcleaning of the filter bags 12. Such a partial cleaning of the filterbags 12 could be accomplished by supplying a lower than normal airpressure from the tanks 34, 36, 38, or by supplying such air pressurefor a shorter period of time. A partial cleaning of the filter bags 12means cleaning at a lower cleaning intensity than normal, and has theadvantage of leaving more sorbent remaining on the filter bags 12, suchthat also after such partial cleaning a certain mercury removal remains.

If the outcome of the check in the second alternative second step 64 is“NO”, then the control unit 48 returns to the first step 60 withoutperforming any cleaning of the bags filter 12, since such cleaning isnot necessary. If the outcome of the check in the second alternativesecond step 64 is “YES”, then the control unit 48 proceeds to a secondalternative third step, denoted 68 in FIG. 2, in which the filter bags12 are cleaned. The second alternative third step 68 represents a fullcleaning, i.e., cleaning at normal cleaning efficiency, resulting inremoval of almost all of the sorbent from the filter bags 12. In such asituation, in which the concentration of mercury HGIN is equal to, orless than, the average mercury concentration HGMEAN, there is no needfor an extra dust cake on the filter bags 12, and, thus, the cleaning ofthe filter bags 12 is controlled based on the pressure drop only.

After having cleaned the filter bags 12 in the third step 66 or 68, thecontrol unit 48 returns to the first step 60.

The method illustrated in FIG. 2 could be applied to the bag house 10seen as a whole, meaning that all compartments 14, 16, 18 are cleaned atthe same time when the control unit 48 proceeds to the third step 66 or68. Often it is more advantageous, however, to apply the methodillustrated in FIG. 2 separately to each of the three compartments 14,16, 18. Hence it could happen, in a situation when HGIN is higher thanHGMEAN, that one of the compartments, e.g., compartment 14, must becleaned, because the pressure drop DP over that compartment 14, asmeasured by the pressure transducer 50, exceeds the second pressure dropset point DPHighHigh. The other two compartments 16 and 18 may, however,not be in need of cleaning, by which is meant that the pressure dropover those respective compartments 16, 18 is lower than DPHighHigh,which means that those two compartments 16, 18 could maintain theirthick dust cakes and maintain a high efficiency in mercury removal.Thus, the decreased mercury removal of the compartment 14, caused by thenecessary cleaning of the filter bags 12 of that compartment 14, couldbe compensated for by the other compartments 16, 18, such that themercury removal of the bag house 10 seen as a whole would besufficiently efficient.

In an alternative embodiment the control unit 48 could receive a signalfrom the second mercury analyser 54, measuring the concentration ofmercury in the cleaned flue gas, i.e., the flue gas in the clean gasduct 26. Such a signal could be utilized by the control unit 48 forcontrolling when cleaning of the filter bags 12 should be made. Forinstance, if the measured concentration of mercury in the clean gas duct26 is substantially lower than the mercury emission limit, then thecontrol unit 48 could control the cleaning of the filter bags 12 basedon the pressure drop DP in relation to the first pressure drop set pointDPHigh, i.e., in a similar manner as indicated in FIG. 2 with referenceto the second alternative of the second step 64. If, on the other hand,the measured concentration of mercury in the clean gas duct 26 is quiteclose to the mercury emission limit, or even higher than that limit,then the control unit 48 could control the cleaning of the filter bags12 based on the pressure drop DP in relation to the second pressure dropset point DPHighHigh, i.e., in a similar manner as indicated in FIG. 2with reference to the first alternative of the second step 62, tomaintain a thick cake of dust on the bags 12 to maintain a highefficiency in the removal of mercury.

FIG. 3 illustrates in a graphical manner the alternative method,described hereinbefore, according to which the control unit 48 controlswhen the filter bags 12 of the first compartment 14 of the bag house 10should be cleaned by means of pulsing them. In a first step, denoted 160in FIG. 3, the control unit 48 has received a signal from the secondmercury analyser 54 about the measured concentration of mercury in theclean flue gas in the duct 26. The control unit 48 checks if themeasured concentration of mercury, HGOUT, is close to, or even higherthan, the emission limit concentration of mercury, HGLIM, which could bea concentration set by, e.g., environmental authorities. If the answeris “YES”, then the control unit 48 proceeds to a first alternativesecond step, denoted 162 in FIG. 2, which has a similar function as thefirst alternative second step 62 described hereinbefore with referenceto FIG. 2. Thus, in a situation when the outlet concentration ofmercury, HGOUT, is close to, or even above, the mercury emission limit,HGLIM, the dust cake is allowed to remain on the filter bags 12 for anextended period of time, to increase the mercury removal capacity of thebag house 10, without having to add more sorbent than under normalmercury concentrations. If the answer in the first step 160 is “NO”,then the control unit 48 proceeds to a second alternative second step,denoted 164 in FIG. 3, which has a similar function as the secondalternative second step 64 described hereinbefore with reference to FIG.2. A third step 166, following the answer “YES” in step 162 or 164,involves a full cleaning of the filter bags 12. As alternative, thecleaning of the filter bags 12 could be performed in different manners,depending on the outcome of the first step 160, in a similar manner,i.e., at a low or a normal cleaning efficiency, as describedhereinbefore with reference to FIG. 2 concerning the third steps 66 and68. After having cleaned the filter bags 12 in the third step 166, thecontrol unit 48 returns to the first step 160.

The method described with reference to FIG. 3 could be utilized forcontrolling the cleaning of the filter bags 12 of the bag house 10 as awhole, or for controlling, separately, the cleaning of the bags 12 ofeach respective compartment 14, 16, 18.

In a still further embodiment the control unit 48 could utilizeinformation from both the first and the second mercury analysers 52, 54.For example, the control unit 48 could control the cleaning of thefilter bags 12 based on the signal from the first mercury analyser 52,and could control the dosage of sorbent from the silo 30 based on athird parameter, which would be based on the signal from the secondmercury analyser 54. In such an alternative embodiment a highconcentration of mercury, i.e., HGIN>HGMEAN, is, first of all, handledby delaying the cleaning of the filter bags 12, in accordance with themethod illustrated hereinbefore with reference to FIG. 2. If this is notsufficient, such that an increased emission of mercury, HGOUT, ismeasured by the second mercury analyser 54, then the dosage of sorbentfrom the silo 30 is increased, to increase further the capacity of thebag house 10 to remove mercury from the flue gas. Hence, the cleaning ofthe filter bags 12 is controlled based on the inlet concentration ofmercury, HGIN, and the dosage of sorbent is based on the outletconcentration of mercury, HGOUT.

FIG. 4 illustrates a schematic example of the effect of the control unit48 controlling the cleaning of the bag house 10 in accordance with themethod described hereinbefore with reference to FIG. 2. For reasons ofclarity the example of FIG. 4 refers to simultaneous cleaning of allcompartments 14, 16, 18, which is not necessarily the best way ofperforming the cleaning, as described hereinbefore. The left y-axisrefers to the concentration of mercury in mg/Nm³ dry gas, and the righty-axis refers to the pressure drop over the bag house 10, in Pascal. Thex-axis is a time scale. Between the times T0 and T1 illustrated in FIG.4 the measured concentration of mercury in the duct 8, HGIN, is lessthan the mercury set point, which is the average concentration ofmercury, HGMEAN, as measured during, e.g., the last 10 days. Thus,during T0 to T1, a full cleaning of the filter bags 12 is initiated, inaccordance with the second alternative third step 68 describedhereinbefore with reference to FIG. 2, each time the pressure drop DPover the bag house 10 exceeds DPHigh. However, at the time T1 thecontrol unit 48 detects, in accordance with the first step 60 accordingto FIG. 2, that HGIN exceeds the mercury set point HGMEAN. In responseto this observation the control unit 48 delays the cleaning of thefilter bags 12, in accordance with the first alternative second step 62illustrated in FIG. 2, until the pressure drop DP over the bag house 10exceeds DPHighHigh. When the pressure drop DP exceeds DPHighHigh apartial cleaning of the filter bags 12 is performed in accordance withthe first alternative third step 66 illustrated in FIG. 2. Such apartial cleaning results in that, as is illustrated in FIG. 4, thepressure drop DP over the bag house 10 is reduced from DPHighHigh toabout DPHigh. Thus, starting at T1, the dust cake is allowed to growthicker on the filter bags 12, until DP exceeds DPHighHigh, and the dustcake is not entirely removed by the partial cleaning.

As can be seen from a reference to FIG. 4 the concentration of mercuryin the clean gas duct 26, i.e., HGOUT, does not increase after T1, inspite of the increased amount of mercury in the flue gas upstream of thebag house 10. Thus, the increased pressure drop over the bag house 10after the time T1 provides a thicker dust cake which increases thecapacity of removing mercury.

At the time T2 illustrated in FIG. 4 the control unit 48 detects thatHGIN has decreased to below HGMEAN. Thus, after the time T2, thecleaning of the bags 12 is again controlled based on the pressure dropDP only, and thus the decision to clean or not to clean the filter bags12 is taken in the second alternative second step 64 illustrated in FIG.2.

It will be appreciated that numerous variants of the embodimentsdescribed above are possible within the scope of the appended claims.

Hereinbefore it has been described that the mercury set point is theaverage inlet mercury concentration HGMEAN. The HGMEAN value couldeither be a fixed value, based on experience, or could be a real longterm rolling average, such as a monthly average of the mercuryconcentration as measured in the duct 8. It will be appreciated thatother mercury set points could also be used. Examples of such mercuryset points include a fixed specific value, not related to the average,such as 10 mg/Nm³, or a value that is related to the average inletmercury concentration, e.g., a mercury set point could be 120% of theHGMEAN value. Furthermore the mercury set point could be calculatedcontinuously based on a mathematical equation. The basis for suchmathematical equation could be parameters like temperature, flue gasflow, sorbent flow, etc. The control unit 48 could also be operative foroptimizing the sorbent supply and the pressure drop DP at which thecleaning of the filter bags 12 is initiated in such a manner thatoverall operating costs are minimized, and mercury emission is keptbelow the emission limit as specified by authorities.

It has been described hereinbefore, with reference to FIGS. 2 and 3,that the cleaning of the filtering surfaces is delayed when an increasedmercury removal efficiency is temporarily desired. With reference toFIG. 2 it has also been described that the cleaning efficiency duringperiods when an increased mercury removal efficiency is desired can belower, see step 66 illustrating a partial cleaning, compared to periodswhen there is no desire for any increased mercury removal efficiency,see step 68 illustrating a full cleaning. In accordance with a furtheraspect of the invention, the cleaning efficiency of the cleaning of thefiltering surfaces could be made lower during periods when an increasedmercury removal is desired, without combining this with a delay of thecleaning. Hence, a desire for an increased removal capacity with respectto mercury could be handled by, as a first alternative being illustratedin FIG. 3, delaying the cleaning of the filtering surfaces, or, as asecond alternative being indicated by steps 66 and 68 of FIG. 2, bycleaning the filtering surfaces at a lower cleaning intensity, or, as athird alternative being illustrated in FIG. 2, by combining the delayingof the cleaning of the filtering surfaces with the cleaning of thefiltering surfaces at a lower cleaning intensity. Thus, the increasedmercury removal at times of high mercury concentrations can be achievedby delaying the cleaning of the filtering surfaces and/or by cleaningthe filtering surfaces at a lower cleaning intensity.

It has been described hereinbefore that the control unit 48 compares afirst parameter in the form of the measured concentration of mercury, asmeasured either upstream or downstream of the bag house 10, to a mercuryset point. The control unit 48 may also be operative for predicting thefuture mercury load, and to control the cleaning of the filter bags 12based on such a prediction. For example, the control unit 48 couldcalculate the derivative of the inlet mercury concentration, HGIN ofFIG. 4. Based on such derivative the control unit 48 would be able topredict that the inlet mercury concentration is about to increaserapidly and to act accordingly, by delaying cleaning of the filter bags12, and possibly by also increasing the supply of sorbent from the silo30. In such a case the mercury set point is a mercury concentrationchange rate, to which the derivative of the measured mercuryconcentration is compared when the control unit 48 is to determinewhether the cleaning of the filter bags is to be delayed or not.Furthermore, a more advanced function could also be utilized forobtaining a value of a first parameter that is indicative of the amountof mercury that needs to be removed in the filter. Such a function couldcalculate a mercury value, for use as said first parameter, based on theabsolute value of the measured mercury concentration and the firstderivative of the measured mercury concentration, and the secondderivative of the measured mercury concentration. Such a function couldhave the general appearance of:Mercury value=F(HGIN(t),d1(HGIN(t)),d2(HGIN(t)))  [eg. 1.1]

The Mercury value calculated by means of the above function, eq. 1.1,could be compared to a corresponding mercury set point. In accordancewith a further alternative a function could be obtained that alsoaccounts for the measured concentration of mercury in the cleanedprocess gas, i.e., in the clean gas duct 26, and the derivative of suchmercury concentration. Such a function could have the general appearanceof:Mercury value=F(HGIN(t),HGOUT(t),d(HGIN(t)),d(HGOUT(t)))  [eg. 1.2]Such a function would thus account for both inlet and outletconcentrations of mercury, and the rate of changes in suchconcentrations. The Mercury value calculated by means of the abovefunction, eq. 1.2, could be utilized as said first parameter and becompared to a corresponding mercury set point. Thus, there are severalways of obtaining a first parameter that is indicative of the amount ofmercury that needs to be removed in the filter. In its simplestapplication the mercury concentration is measured upstream or downstreamof the filter and is, in the form of a first parameter, compared to afixed mercury set point. In more advanced applications the mercuryconcentration is measured both upstream and downstream of the filter anda first parameter in the form of a mercury value is calculated based ona complex function, such as that of eq. 1.2, and is compared to amercury set point that in itself could be calculated based on anothercomplex function. A very simple equation for calculating a firstparameter in the form of a mercury value is given in eq. 1.3:Mercury value=HGIN+HGOUT*constant K  [eq. 1.3]The constant K of eq. 1.3 could typically be 5-15. A first parametercalculated in accordance with eq. 1.3 could be compared to a mercury setpoint being the average inlet mercury concentration times a factor 2.Hence, when the first parameter, calculated in accordance with eq. 1.3,is larger than HGMEAN*2, then the cleaning of the filter bags 12 shouldbe delayed, and/or made at a lower cleaning intensity.

It has been described, with reference to FIG. 4, that the pressure dropDP over the bag house 10 is measured, and is utilized for controllingwhen the filter bags 12 should be cleaned. It will be appreciated thatother parameters may also be measured for the purpose of establishingwhen cleaning of the bags 12 is required. One such example is the filterflow resistance in Pa s/m, which can be calculated from the ratio of thesignal for the measured filter pressure drop and the signal for themeasured gas flow. According to a further alternative simple timerscould be utilized for controlling when the cleaning of the filter bags12 should be initiated, provided that the operating conditions of thebag house 10 are quite stable as regards flow of process gas, and amountof material collected per unit of time. Thus, for example, when theamount of mercury is low, the bags could be cleaned every 30 minutes.When the amount of mercury increases above the mercury set point thecleaning of the bags could be made only every 45 minutes, to increasethe mercury removal efficiency of the bag house 10 by forming thickerdust cakes.

Hereinbefore it has been described that the filter is a bag house 10. Itwill be appreciated that other types of fabric filters, that do not havefilter bags, may also be utilized. For instance, such fabric filterscould have fabric pockets or flat walls of fabric, through which theprocess gas has to pass. Still further, it would also be possible toutilize other types of filters, such as electrostatic precipitators,such as that described hereinbefore with reference to U.S. Pat. No.4,502,872, or cyclones. Usually, however, fabric filters are preferred,since a dust cake, through which the process gas has to pass, is easilybuilt up on such a filter. As described hereinbefore, influencing thethickness of the dust cake, by delaying the cleaning of the filteringsurface and/or by performing the cleaning at a lower cleaning intensity,when needed in view of the amount of mercury that needs to be removed inthe filter, has a large effect on the mercury removal capacity of thefilter.

To summarize, a method of removing mercury from a process gas by meansof a sorbent and a filter 10 involves applying said sorbent to at leastone filtering surface 12 of the filter 10. A first parameter HGIN;HGOUT, which is indicative of the amount of mercury that needs to beremoved in said filter 10, and a second parameter DP, which isindicative of the amount of material that has been collected on saidfiltering surface 12, are measured. The measured value of said firstparameter HGIN; HGOUT is compared to a mercury set point HGMEAN. Whensaid measured value of said first parameter HGIN; HGOUT is higher thansaid mercury set point HGMEAN, the cleaning of said filtering surface 12is delayed, compared to the point in time suggested by the measuredvalue of said second parameter DP, and/or is performed at a lowercleaning intensity.

While the invention has been described with reference to a number ofpreferred embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Moreover, the use of the terms first, second, etc. do not denoteany order or importance, but rather the terms first, second, etc. areused to distinguish one element from another.

The invention claimed is:
 1. A gas cleaning system for removing, atleast partly, mercury from a process gas, comprising: a control unit tocontrol when a filtering surface of a filter of said gas cleaning systemis to be cleaned of material collected thereon, said control unitfurther to receive a first signal of a measurement from upstream of thefilter of at least one first parameter indicative of an amount ofmercury to be removed in said filter and a second signal of ameasurement of at least one second parameter indicative of an amount ofmaterial collected on said filtering surface, utilized for determiningwhen the filtering surface should be cleaned, the control unit stillfurther to compare a measured value of said first parameter to a mercuryset point, to delay, when said measured value of said first parameterindicates an amount of mercury higher than an amount of mercuryindicated by said mercury set point, the cleaning of said filteringsurface as compared to a point in time suggested by a measured value ofsaid second parameter, and to clean the filtering surface at a selectedintensity when the measured value of the second parameter exceeds afirst pressure drop set point when the measured value of the firstparameter indicates an amount of mercury equal to or lower than theamount of mercury indicated by the mercury set point, and when thesecond parameter exceeds a second pressure drop set point with a higherpressure drop than the first pressure drop set point when the measuredvalue of the first parameter indicates an amount of mercury higher thanan amount of mercury indicated by the mercury set point, for increasedmercury removal.
 2. A gas cleaning system according to claim 1, whereinsaid mercury set point is a concentration of mercury in the process gasupstream of the filter measured by a first mercury analyser, suchconcentration related to said first parameter.
 3. A gas cleaning systemaccording to claim 1, wherein said mercury set point is related to aconcentration of mercury in the process gas downstream of the filtermeasured by a second mercury analyser, such concentration related tosaid first parameter.
 4. A gas cleaning system according to claim 1,wherein said second parameter is a pressure drop over the filter, andthe control unit initiates cleaning of the filtering surface when saidsecond parameter exceeds a first pressure drop set point, and initiatescleaning of the filtering surface when said second parameter exceeds asecond pressure drop set point, which refers to a higher pressure dropthan the first pressure drop set point.